The present invention relates to a method for determining the speed variation between wheels of different vehicle axles, and, more particularly, to a method of forming a difference between speeds of rear wheels and speeds of front wheels, determining a correction which compensates for the variations between the speeds of the front wheels and the rear wheels arising from vehicle geometry, and providing to a vehicle system the speed variation between the wheels of the separate vehicle axles in terms of the difference plus the correction.
A method is disclosed in DE 35 05 455 A1 which gives an equation by way of which the speed variation of the rear wheels relative to the front wheels is determined. In particular, the difference is first determined between the speed of the rear wheels and an average speed of the front wheels. This known method also takes account of understeer and oversteer behavior when the vehicle is travelling around a curve by introducing a correction value which is referred to as the Ackermann correction which is a function of the difference between the front-wheel speeds, which is multiplied by the speed of the rear wheels. The speed variation DV of the rear wheels relative to the front wheels is then determined from the following equation: EQU D=VHM-VVM+VHM*f(DLA)
where VHM is the average speed of the wheels of the rear axle wheels, VVM is the average speed of the front-axle wheels, DLA is the difference between the speeds of the front wheels and f(DLA) is the function used for making the correction.
In the determination of wheel slip, it is also known to take account of the situation when the vehicle is travelling around a curve (as discussed in DE 39 23 782 A1), but in this method, the Ackermann correction is not made and account is only taken, for each axle individually, of the higher speed of the wheel on the outside of the curve due to the fact that the vehicle is travelling around the curve.
Relative to this state of the known art, it is an object of the present invention to improve the applicability and mode of operation of the above-described method. This object has been achieved in accordance with the present invention by providing that under particular driving conditions which are characterized at least in that acceleration of the rear wheels is below a threshold value and that the vehicle is travelling around a curve, such that no speed difference of the wheels of separate vehicle axles due to drive slip exists, an adaptation of the correction takes place by determining the difference between the speeds of the rear wheels and the front wheels and by adapting the correction so that a magnitude of the correction, under the particular driving conditions, is equal to the difference between the speeds of the rear wheels and the front wheels and the correction exhibits a change of sign relative to this difference and also further that in the step of determining the correction, transverse vehicle acceleration is taken into account by magnitude of the correction becoming smaller with increasing transverse acceleration.
An advantage of the present invention resides in the fact that both the applicability and mode of operation of the method are improved in a combination of two embodiments.
In the method according to the present invention which is employed in conventional electronic equipment associated with known sensors for determining wheel speeds, it is not only the speed differences resulting from the vehicle geometry which are taken into account in a particularly advantageous manner, but also those speed differences which arise because of the vehicle dynamics are accounted for. With increasing transverse acceleration, the path radii of the front and rear axles approach one another because of the changing drift angle. This means that because of the vehicle dynamics, there should be no correction in the case of large transverse accelerations because vehicle dynamics provide compensation for the effect of the vehicle geometry.
In what follows, the relationships are described for a vehicle which has a driven rear axle in normal driving operation. The present invention can be used in a particularly advantageous manner in all vehicle systems in which the slip between the axles of the vehicle is evaluated for the fulfillment of their function, such as automatically switching on a limited-slip differential between the axles of the vehicle or automatically selecting all-wheel drive in the case of slip control (acceleration skid regulation ASR). All the speeds should, furthermore, be considered in terms of km/h units where orders of magnitude are given.